All-terrain vehicles (ATVs) are designed to traverse rugged terrain. Accordingly, vehicle stability is one of the primary design considerations. As is well known in the art, vehicle stability can be improved by lowering and centralizing mass without unduly compromising ground clearance and ergonomics. Mass centralization can be improved by locating heavy components as close as possible to the geometrical center of the vehicle.
In the prior art, some ATV manufacturers (e.g. Yamaha and Kawasaki) have developed drivetrains in which the subtransmission is separated from the engine. These have benefits in terms of assembly and maintenance. For example, U.S. Pat. No. 6,286,619 (Uchiyama et al.) discloses an ATV transmission in which a final drive assembly is mounted to a rear of the frame and is operatively connected to the engine via a belt or chain. This drivetrain design expedites assembly by obviating the need to install a fully assembled drivetrain (engine and subtransmission). Likewise, in U.S. Pat. No. 6,601,668 (Kitai et al.), a rear reduction gear case is mounted at the rear of the ATV and receives power from the engine via a rear propeller shaft.
However, these prior-art drivetrains are suboptimal in terms of mass centralization as the substantial weight of each subtransmission is located toward the rear of the vehicle.
Furthermore, as is known in the art, engine-generated reaction forces are borne by the frame at the engine-mounting points. As the subtransmission is mounted at a distance from the engine, all engine reaction loads are concentrated at the few connection points where the engine casing is joined to the frame.
In light of the foregoing, there remains a need for an ATV drivetrain that improves vehicle dynamics by redressing at least one of the aforementioned deficiencies of the prior art.